1. Field of the Invention
The invention relates in general to a device used in a photolithographic integrated circuit fabrication process, and more particularly to a photoresist solution storage and supply device that can reduce photoresist solution waste.
2. Description of the Related Art
Photolithography has become one of the most important techniques in the semiconductor manufacturing industry. For instance, various patterns on thin film component layers, as well as areas for implantation of impurities, are defined using photolithographic processes. In fact, the ability to increase the level of integration of semiconductor components is related to whether component dimensions can be reduced, and dimensional reduction depends very much on the progress of photolithographic techniques.
A photolithographic process involves first coating a layer of photosensitive material, commonly called photoresist, over the silicon chip, and then, with an intermediate mask in place, exposing the layer to a light source. The mask is composed of light penetrable and non-penetrable areas arranged so that a pattern can be defined in the photoresist layer. Areas in the photoresist layer that have been exposed to light can be characterized as either soluble or insoluble types, according to the characteristic changes they undergo through reaction with developing solution. If, after exposure to light, the exposed area is soluble in the developer, leaving behind the unexposed area, then such photosensitive material is called positive photoresist, and the pattern so defined is the same as the one in the photomask. On the other hand, if after exposure to light, the exposed area is insoluble in the developer and the unexposed area is soluble, then this kind of photosensitive material is called negative photoresist, and the pattern so defined is complementary to the photomask.
Thus, a photolithographic process basically consists of three major steps, namely, photoresist coating, light exposure, and developing.
A conventional photoresist storage and supply device as shown in FIG. 1 includes a manually operated valve 8, a photoresist solution storage vessel 14, a liquid level sensor 16, a pump 20, a filtering device 24, gas pipelines 6, 12, 10, and liquid pipelines 18, 22, 26. A photoresist solution 15 is pumped by the pump 20 from the photoresist solution storage vessel 14 through the pipeline 18 until the liquid level sensor 16 has detected a predefined low liquid level situation, in which case the empty vessel needs to be replaced. The photoresist solution 15 from the pump 20 is sent via pipeline 22 to a filtering device 24, and then is passed on through the pipeline 26 into subsequent processing stages (for example, constant temperature setting) before a photoresist coating operation. A gas 5 (for example, nitrogen) is passed from the pipeline 6 through the manually operated valve 8 into the pipeline 10 and then enters into the photoresist solution storage vessel 14. Lastly, it is expelled through holes 24a in the filtering device 24.
The aforementioned conventional photoresist solution storage and supply device has several drawbacks, including the following:
(1) The liquid level sensor is hard to adjust and can easily lead to either waste caused by too much photoresist solution still remaining inside the vessel when it is replaced, or gas bubbling into the pipelines caused by over-extraction of photoresist solution, resulting in defective photoresist coatings on silicon chips.
(2) Machine operating time is wasted by replacing used photoresist solution storage vessels, as the machine must stop just to change the vessel.
(3) The gas valve must be operated manually to let gas in, forcing the photoresist solution to flow into the pipeline leading to the pump, and the gas is expelled through the exhaust holes in the filtering device.